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Water potential

You are what you eat

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Carbohydrates

Contents

• Monosaccharides.
• Sugar derivatives.
• Polysaccharides.

Monosaccharides

Monosaccharides are the building blocks of larger sugars, and their biochemistry is possibly the most complex and confusing of all the biochemicals. Here is a brief list of the key terms in monosaccharide nomenclature for glucose:

• Number of carbons: six, hence hexose.
• Numbering of carbons: C1 is the carbonyl carbon, C6 the farthest carbon from this.
• Type of carbonyl compound: terminal CHO (aldehyde) hence aldose.
• D/L descriptor: the OH is to the right on the farthest chiral carbon from the CHO group (i.e. C5) in the Fischer projection, and in the Haworth projection, hence D-glucose.
• Names of sugars: the three intermediate chiral carbons (C2, C3, C4) indicate that this is glucose.
• Ring forms: six membered ring with anomeric OH (C1) on same side as CH₂OH group (C5 and C6), hence β-pyranose.

The number of carbons in a sugar determines which size group of sugars it belongs to:

• 3 carbons : triose.
• 4 carbons : tetrose.
• 5 carbons : pentose.
• 6 carbons : hexose.
• 7 carbons : heptulose.

Open chain monosaccharides (like the Fischer projection above) have n-2 chiral carbons, so 2^n-2 isomers (assuming no internal symmetry). Note that sugars can also react internally to produce a variety of rings forms: this introduces an extra chiral centre, as you can see in the Haworth projection above.

These are a hexose and a triose.

We number the chain from the CHO group at the top down to the CH₂OH group at the base (i.e. number from the end nearest the carbonyl group). This way of drawing monosaccharides is called a Fischer projection: imagine the vertical bonds curling away from you and the horizontal bonds sticking out at you, like the back of a stegosaurus.

The type of carbonyl group present in a sugar is an important determinant of its chemical properties. Sugars with a terminal CHO group are chemically aldehydes, and are termed aldoses e.g. glyceraldehyde. These sugars are 'reducing sugars': they reduce the blue Cu²⁺ in Benedict's solution to a red precipitate of Cu⁺. This is the test for reducing sugars so beloved of A-level syllabi. Sugars with an internal CO group, R-CO-R are ketones, and are called ketoses e.g. dihydroxyacetone (DHA). Although ketones are not generally reducing agents, the OH attached to the next carbon along is capable of reducing Benedict's reagent, hence all monosaccharides are 'reducing sugars'.

Sugars have a somewhat arcane nomenclature. So far it has been mostly logical, but from now on, it's largely arbitrary decisions made fifty years ago or more. Apologies for this.

Sugars contain many chiral centres. Naming these chirals centres should be easy (just use R or S descriptors), but in fact, there are three systems for different bits of the sugar molecule. The first descriptor is used to name the chiral carbon farthest from the carbonyl group. This is called the D/L descriptor. If you draw a sugar it in the Fischer projection (as above), an OH to the right on the farthest chiral carbon is the D-form, and an OH to the left is the L-form. D-glyceraldehyde appears above. Note that DHA is nonchiral (it's about the only sugar that is!). The two forms of glucose are below, note that they differ at every chiral centre:

The D-isomer of glyceraldehyde also happens to be the R isomer using CIP rules, as you can see from the diagram below.

It also happens to be the (+) isomer from polarimetry. This is easy to remember (R, D and +), but only by fluke! The remaining chiral centres in an open chain monosaccharide get 'proper' names. These are the D-forms of the common monosaccharides. Note that they differ only at carbons 2, 3 and 4.

Note that D-galactose and D-glucose differ at only one chiral centre: such a relationship makes them epimers.

The third descriptor for monosaccharides only comes into play when sugars internally react to form rings. Monosaccharides react internally by nucleophilic attack of the carbonyl group by a OH group, to form a cyclical hemiacetal:.

Two chiral ring forms are possible, which introduces a new chiral center, so we now have (maximally) 2^n-1 isomers. These diastereomers are called the α and β anomers. The α anomer has it's 'new' OH on the opposite side of the molecule to the CH₂OH group. The β anomer has it on the same side, as you can see below.

For glucose solutions, typically, the ratios of forms are 36% α, 63% β, <1% open chain. When you dissolve glucose in water, the slow conversion of the solid equilibrium mixture to the aqueous equilibrium mixture means that the rotation glucose causes to polarised light decreases (becomes less positive) over time. This is called mutarotation.

A final complication for the ring forms is that the rings of hexoses and pentoses can be either 5 or 6 membered (including the heterocyclic oxygen atom), depending on which carbon (usually 4 or 5) the CO group is attacked by. Five membered rings are called furanoses, after the chemical furan. Six membered rings are called pyranoses, after pyran. These are glucopyranose and glucofuranose.

The Haworth notation has been used above to show the relative positions of the OH groups in the ring form. Haworth notation is a simple and tidy way of representing ring forms, and is fairly self explanatory. Sugars actually take up cyclohexane shapes in the pyranose form, as shown below for glucose, but Haworth notation is considerably easier to interpret.

To convert between Haworth and Fischer projections is a little tricky. Remember that the Fischer projection curves away from you, with the OH and H groups sticking out like the plates on a stegosaur's back. If you turn the Fischer projection clockwise through 90° in your mind's eye, you will see that for carbons 2, 3, and 4, if the Fischer OH goes right, the Haworth OH goes down. Carbon 1 is the anomeric carbon, and doesn't exist in the open chain form, so we're left with carbon 5 to understand. You need to realise there is free rotation of the H, OH and CH₂OH groups around carbon-5, hence you can twirl the three groups round so that the CH₂OH is pointing up as you curl the chain round into a ring. If this is too much for you (and I don't blame you!), you might want to play with a ball-and-stick kit, or make up the C-5 stereochemistry using a piece of BluTak, four matchsticks and some sticky labels. It's what I did!

Sugar derivatives

You may be glad to know that's all there is to monosaccharides. However, there are endless compounds that themselves contain monosaccharides, not least the polysaccharides, but we will take a brief diversion first through the world of sugar derivatives.

An ester is a compound created by reacting an alcohol with an acid. Sugars are alcohols, hence they can form esters with organic acids, and other common biochemical acids. Phosphate sugar esters are common cofactors in enzymes: Coenzyme A, ATP, cAMP, UTP, etc. are all phosphate esters of ribose derivatives:

DNA and RNA are also obviously built from sugar phosphate derivatives of ribose and 2-deoxyribose. Sugars also form carboxylate esters including the glycoside glucogallin, found in rhubarb.

We just mentioned 2-deoxyribose. This is an example of a deoxysugar, i.e. a sugar with an H where it 'should' have an OH.

Polyols (sugar alcohols) are a further example of sugar derivatives. They have no carbonyl group: they have R-CH₂OH instead of R-CHO. They are usually named after the sugar they are derived from.

• Xylose gives xylitol (used in sugar free mints).
• Sorbose gives sorbitol (in sugar free mints and cherries).
• Glyceraldehyde (glycerose?!) gives glycerol (in membrane lipids).

Sugar free mints contain polyols that are indigestible to humans and tooth decay bacteria, so they are good for teeth and slimmers, but excessive consumption leads to diarrhoea, because the excess indigestible polyol makes osmosis drag water into the colon from the blood. Flatulence is also possible, as some bacteria in the gut can ferment polyols.

You can oxidise the aldehyde group of sugars to produce carboxylic acids. A common carboxylate found in extracellular matrix polymers is gluconic acid, which is produced from glucose by reaction with oxygen (with hydrogen peroxide as a byproduct).

Amino derivatives of sugars include β-D-N-acetylglucose-2-amine, the building block of chitin.

Sugars can both both derivatised, and reacted with other compounds to form glycosides. Glycosides are held together by a glycosidic bond, which is basically an ether linkage: R-O-R. Ether (the old anaesthetic) itself is CH₃CH₂-O-CH₂CH₃. If the other compound is just another sugar, the glycoside is a polysaccharide. If it's something else, then the glycoside is a glycoside proper, which we'll discuss briefly now. The 'not sugar' involved in a 'proper' glycoside is called the aglycone.

Sugar-OH + Aglycone-OH → Sugar-O-Aglycone + H₂O.

Sugar + Aglycone → Glycoside + Water.

In fact, the aglycone doesn't necessarily have to be an alcohol. It can also be a thiol or amine:

Sugar-OH + Aglycone-XH → Sugar-X-Aglycone + H₂O.

X may be N (amine), O (alcohol) or S (thio), producing N, O, and S glycosides respectively. An O-glycoside is a conventional ether. Plants often contain N- and S- glycosides too. Glycosides are often more soluble than their aglycones (because of all the OH groups), and are often involved in the excretion of water insoluble toxins like DDT in insects. The cyanogenic glycosides, which are antifeedants in cabbages and bracken, are stored in the plant's vacuole. When the cell is damaged, the vacuole mixes with the cytoplasm, which contains glycosidases, which release cyanide from the glycoside so killing herbivores. Many other glycosides are toxic: digitoxin is a good example here. This is a glycoside from foxgloves (Digitalis purpurea) formed between a sugar and a steroid.

Cardiac glycosides like digitoxin stimulate heart beat (by affecting the K/Na pumps in the heart), and are used for dropsy and arrhythmia. A lethal dose is just three leaves. You can see the three sugars and the aglycone (digitoxigenin) in the diagram above.

It may not be that obvious, but ATP is also a glycoside (as well as being a sugar ester): the adenine is linked to the sugar via an N-glycosidic bond (ribose-OH + adenine-NH → adenine-N-ribose + water).

Polysaccharides

Polysaccharides are glycosides between sugars. The name given to the polysaccharide is dependent on the size of the molecule:

• 1 sugar unit: monosaccharide
• 2 sugar units: disaccharide
• 3 sugar units: trisaccharide
• 3-several sugar units: oligosaccharide
• 100+ sugar units: polysaccharide

The links between the sugar molecules fix the orientation of any anomeric carbons involved in the bond, and the glycosidic bonds can form between any two alcohol groups on the sugar, making polysaccharide nomenclature a nightmare. Probably the simplest and most important disaccharide is sucrose. Chemically, sucrose is β-D-fructofuranosyl-(2→1)-α-D-glucopyranoside. The D-fructofuranose and D-glucopyranose bits should be obvious. The rest is a little more complex. Firstly, note that for convenience, the Haworth notation for fructose is upside down (carbon-1 is on the left, not the right, of the diagram). Both sugars are numbered for your convenience. Glucose is in the α anomeric form (OH on carbon-1 is on the opposite side to the CH₂OH on carbon-5), and fructose is in the β anomeric form (OH on carbon-2 is on the same side as the CH₂OH on carbon-5). These configurations are fixed because both the anomeric carbons are involved in the glycosidic bond. The last thing to note is that the bond goes from carbon-2 on fructose to carbon-1 on glucose, hence the 2→1. The IUPAC preferred way of naming disaccharides that have fixed anomeric carbons are as 'glycosyl glycosides', with glucose preferred as the glycoside portion rather than the glycosyl in this case. It's all a bit arcane, and you'll often find sucrose 'wrongly' (for some value of 'wrongly') called α-D-glucopyranosyl-(1→2)-β-D-fructofuranoside, or similar.

Simple, eh? Hmmm. One further thing to note is that sucrose is a 'non-reducing' sugar. All monosaccharides are reducing sugars by virtue of having either an aldehyde group or a -(C=O)-CHOH- group, both of which can react with Benedict's reagent. Many disaccharides are also reducing because they exist in equilibrium with an open-chain form, much like glucose mentioned above. Sucrose is different because the the glycosidic bond forms between C-1 of glucose and C-2 of fructose, which are the very OH groups formed when these sugars cyclise. The formation of a glycosidic bond between these two anomeric OH groups prevents the sucrose molecule from having an open-chain form, hence sucrose is unable to participate in reduction of Benedict's reagent.

There are a number of important large polysaccharides. Most of them are either food storage or structural polymers. Of the structural polymers, the most important and cellulose and chitin. Both are glucans (i.e. homopolymers of glucose units). All the bonds go from (1→4), and as all the glucose units are the same, and all are held in the β anomeric form, they are both termed β-(1→4)-glucans. Cellulose is simply that, but chitin is made from the modified sugar N-acetylglucosamine, making it β-(1→4)-poly-N-acetylglucosamine, or something. The way in which the bonds hold the cellulose and chitin molecules leads to their being able to form strong intramolecular hydrogen bonds between separate glucan chains. This makes them ideal for building large, strong fibres, and this is indeed what they are used for: cellulose forms the cell wall of plants and some algae, and chitin forms the carapace of insects and other arthropods.

The food storage polysaccharides are usually α-(1→4)-glucans. This slight change in the manner of linkage makes the molecules helical rather than long, straight 'bars', like cellulose, and they are able to form their hydrogen bonds with water, making them somewhat more soluble. Amylose/amylopectin and glycogen are the main animal and plant storage carbohydrates. Amylose is simply a α-(1→4)-glucan, but glycogen and amylopectin are branched with α-(1→6) linkages too.

Plants also contain more exotic polysaccharides in their cell walls, including pectins and hemicelluloses. Animals likewise have hyaluronan, and other glucosaminoglycans (GAG) in their extracellular matrices. GAGs and pectins are sugar/sugar-acid/sugar-amine polymers, and hemicellulose is a small polymer of glucose with variable amounts of rhamnose, galactose, xylose, mannose, fucose, and others. Their structures are somewhat complex!

Test yourself

1. Identify the following monosaccharide as a D or L sugar, as an aldose or ketose, as a hexose, pentose or tetrose, as an α or β anomer, and as a pyranose or furanose ring.

   

2. The O-glycoside arbutin is extracted from strawberry trees of the genus Arbutus. What are its glycone and aglycone precursors?

   

3. Identify the sugar moieties and linkages in the polysaccharide verbascose:

   

4. These are the sugars D-glucose and L-gulose. What is the relationship between D-glucitol and L-gulitol?

   
   D-glucose

   
   L-gulose

Answers